1. Technical Field
The present invention relates to an image display apparatus and a control method therefor and, more specifically, to the technology for an image display apparatus that displays images through scanning by beam lights modulated in accordance with an image signal.
2. Related Art
The image display apparatus of a recent type for image display is a laser projector, which displays images through scanning by laser lights. The laser lights have characteristics of being highly monochrome and directive so that the images to be displayed by the laser projector can be advantageously high in color reproducibility. The laser projector displays images through modulation of laser lights using rectangular pulse-like modulation currents, for example. As an example, refer to Patent Document 1 (JP-A-2001-189520).
Patent Document 1 describes the technology of modulating laser lights through pulse width modulation. Assuming that a fully high-definition image of 1920 by 1080 pixels is displayed with 30 frames per second, the pixel clock representing the timing for pixel scanning using laser lights will be 62.2 megahertz (MHz). This thus requires a modulation frequency of 16 gigahertz (GHz) for a tone representation of 8 bits. For such high-speed modulation, the configuration is complicated to drive a light source, resulting in high manufacturing cost. Especially a laser light source of high output has a great difficulty in precisely driving with the modulation frequency as high as 16 GHz, thereby possibly reducing the modulation precision and apparatus reliability. When it is difficult to increase the modulation frequency as such, a plurality of laser lights may be used for a tone representation to reduce the modulation frequency of the respective laser lights. If this is the case, however, using 10 laser lights for a tone representation merely achieves 10-fold increase for the minimum pulse width, and this is considered not enough for reduction of the modulation frequency.
Patent Document 1 also describes the technology of adjusting the laser lights by light amount to be proportionate to a power of 2, and selecting the laser lights to suit the number of tones, i.e., the number of quantized bits, for display. The technology uses a light source drive pulse signal synchronous to a pixel clock so that the modulation frequency can be reduced. For an 8-bit tone representation, for example, 8 laser lights are allocated bits of 0 to 7, respectively. The laser light allocated 7-bit being the maximum number of bits will have the light amount equivalent to 128 tones. In this case, the maximum number of tones displayable using 8 Laser lights will be equivalent to 255 tones in terms of light amount. It means that even with 8 laser lights, the resulting light amount will be less than the value of doubling the light amount of a laser light allocated the maximum number of bits. If with 10 laser lights, the resulting light amount will be much less than the doubling the light amount of the laser light allocated the maximum number of bits. Such a previous technology has a difficulty in being able to offer bright images with a simple and reliable configuration.